An oldie, but a goodie

Last Friday, the Anarchist Guild Social Committee – a sketch comedy group of whom I was a member – celebrated their second birthday, and Nick, their leader, asked me if I’d come and contribute to the shenanigans. I said yes, he said he didn’t want a sketch, and after a while I settled on the idea of just doing a science demonstration. While I thought of a few candidates, I settled on that oldie-but-goodie, the egg in the bottle trick, made famous here in Australia by Julius Sumner Miller; you can find a collection of clips from his most famous series, Why Is It So?, on the ABC Science web site. (They’re well worth a watch, though it’s interesting to note that the kind of interaction he has with school students is probably not allowed these days, and there’s a noticeable lack of women and girls on the program – despite his inclusive welcome of “ladies and gentlemen, boys and girls”.)

Of course Why Is It So? is quite an old vintage as television programs go, besides which the egg in the bottle experiment doesn’t appear in any of those clips. Members of my generation (X, in case you were uncertain) probably know the demonstration better from a series of Cadbury chocolate commercials in which Julius appeared in the 1980s – a cunning move, and proof that he was ready to spread the word of science in any way possible! Perhaps I should be seeking that road to success?

Anyway, it’s a classic, and I was genuinely excited to peform it again; it formed a brief part of my first solo science show, Listen to the Man in the Lab Coat in 2004, and served as an encore to my Comedy Festival debut, Science-ology, in 2007. Not having performed it for three years made it just as much a treat for me as for (hopefully) the audience – and given that some of them had never seen it before, I think I picked just the right “science trick” for the crowd. I hope to get some video of my demonstration and put it up here in the near future, but in the mean time, here’s a basic run-down.

First, you need a bottle – it used to be easy to get glass milk bottle, but now they’re a bit of a rarity in Australia. So much so I brought one back from my trip to New York, from the Milk Thistle organic dairy farm. (I’m aware that recent articles say there’s no nutritional benefit to organic farming, overturning previous research, but there are still other advantages, not least the taste. But I digress.) You can find glass milk bottles locally, though, and I recommend you do; other bottles can do the trick, but milk bottles have the perfect sized opening.

Next, you need an egg. The egg should be too big to fit in the bottle, and the larger the better; I used “XL” size (free range) eggs, about 60g each. There are some misconceptions about the egg; I’ve had a number of people complain that they always thought the egg was raw, but it’s always been a hard-boiled, shelled egg for this experiment. (You could try a raw one, and probably get a cool result, but an exploding raw egg is much messier. Actually, now I really want to try that…)

Finally, you need some paper and matches. This is the dangerous bit, so allow me to put in an obligatory warning: don’t do this at home without adult supervision, kids.

Okay, so the egg is too big to go in the bottle, right? How to get it in without mashing it? Set fire to a small strip of paper, drop it in the bottle, then put the egg back on top. After a short time, the egg will be magically “sucked” in to the bottle with a very satisfying pop! How did this happen? Or rather: why is it so?

I have to explain, of course, because that’s the primary difference between a science trick and a magic trick: there’s no scientist’s circle making sure I never explain how it’s done, and in this case, it’s all done with pressure.

(Sometimes someone claims the explanation is “Bernoulli’s principle”, and technically that’s accurate in some sense, though Bernoulli’s principle is specifically about the relationship between a fluid’s speed of flow and its pressure; an aircraft wing is a better example of that.)

So what happens to the egg? First, the flame heats up the air inside the bottle; since this causes the air to expand, there is a greater volume of air than can fit inside, and some of the air escapes by pushing past the egg. (You can often see this if you put the egg on quickly after dropping in the flame; it bounces up and down making farting noises.) Once the flame goes out – not because it has “used up all the Oxygen”, by the way – the air begins to cool, and as it cools, it condenses, decreasing in volume. There’s now a lesser volume of air in the bottle than is needed to fill it, creating an area of low pressure. The higher pressure air outside the bottle exerts a force on the lower pressure air inside – not enough to break the bottle, of course, but enough to push the egg inside until the air can get in and equalise the pressure.

So that’s it! I use a big egg to get a good noise, which means it can’t be extracted easily, though you should be able to do the experiment in reverse by holding the bottle upside down with the egg near the opening and either heating the air inside (to make it expand) or blowing extra air into the bottle past the egg. My eggs are usually too big for this method to be reliable. If you want to get really fancy, you can soak the egg in vinegar without taking the shell off, which makes the shell soft; once you get the egg inside, it will eventually harden, and that’s pretty impressive. If you don’t want tell-tale signs of burnt paper inside the bottle, you can either heat up the air by immersing the bottle in really hot water, or directly reduce the pressure by immersing the bottle in really cold water, which are less reliable and less impressive, but still work.

So there you have it: the egg in the bottle. If you’d like to see me do it, along with all the witty banter I’ve developed, by all means get in touch – consider me available for weddings, parties…anything!

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2 comments

Getting the egg out of the bottle intact is not as easy as getting it in. Obviously a vinegar/bicarb mixture can be used to push the egg out. The problem is that the egg tends to burst coming out. However the egg does not burst going in(Why? The tapered walls of the bottle allowed the egg to expand gradually without tearing.) The solution to the problem is to use a second milk bottle cut in half and taped to the neck of the first bottle. The vinegar mixture is poured in through the top and the whole assembly inverted over a basin of water.NOTE I worked this out over 20 years ago.